Astrobiology Top 10: The Oldest Signs of Life on Earth
In 2008, Nora Noffke was visiting the Dresser Formation in Western Australia, home of Earth’s oldest surface rocks, when she noticed something peculiar on the ground.
“We were just there as geo-tourists, not for work purpose,” says Nora Noffke, a geobiologist at Old Dominion University. “I had been walking with a researcher friend, who wasn’t involved in the study. Then I saw these rocks with the familiar wrinkle marks.”
Noffke is a world-expert on microbially-induced sedimentary structure (or MISS). These form when microbial mats interact with sediments in shallow water. They are found all over the worlds–in lakes, river, lagoons, and shallow marine environments. They can spread out anywhere from a few centimeters to over many kilometers. Evidence for them include microbial mat chips and wrinkle structures.
The Dresser Formation, which dates from the early Archean and is about 3.5 billion years old, has been studied extensively. But these MISS had not been found before. “People didn’t see them because they didn’t know what to look for,” Noffke said.
Noffke later came back with a team. The problem, of course, was to confirm whether these structures were indeed of biological origin. Noffke had developed a set of criteria to distinguish MISS, the product of microbe life, from mere geological phenomena. They include looking at shapes, micro-textures, and chemical composition. Noffke and her team had tested the criteria in previous studies comparing ancient MISS from other sites to modern MISS.
The MISS found in the Dresser Formation did fit the criteria, making them evidence for some of the earliest signs of life on Earth. The results of the study are published this month in the journal Astrobiology.
Prior to this study, the oldest MISS had been found in the Moodies Group of South Africa and dated to 3.2 billion years old. So the new discovery advances the age of the earliest MISS by 300 million years.Other signs of ancient life had been found in the Dresser Formation, in particular stromatolites. Stromatolites and MISS are related, but they’re very different. They both form when microbes interact with sediments. But stromatolites grow upward in the shape of domes or columns, a process that involve carbonate precipitation, while MISS remain planar.
What’s more, MISS have remained unchanged over the last 3 billion years, and they’re believed to be the older relatives. (Ancient stromatolites, on the other hand, are unlike modern ones.) “The MISS of the Dresser Formation look identical to their modern counterparts,” says Noffke. “I was really astonished by how similar they are.”
“But it also raised the question: why are they so identical?” she adds. “And what does that mean about the organisms that created them?”
According to Noffke, the Dresser Formation MISS may have been formed by the same microorganisms found in modern MISS. “Modern microbial mats can serve as an analog model for the interpretation of ancient ones,” she says. Modern microbial mats are dominated by cyanobacteria, which means these microbes (or their earliest relatives) may have already been present on Earth 3.5 billion years ago.
Also of significance is that the conditions early on Earth and early on Mars were very similar. MISS of the Dresser Formation were found in a sabkha environment–a crusty and flat area where the sea water has evaporated or been dried out by the wind. Environments like these indicate the former presence of fluid water and are very common on Mars. This make MISS a great target for the Mars Exploration Rover Program, and for our search for life on other planets.